Goto Start -> Run -> taskschd.msc. Create a basic task and set the trigger to "When the computer starts". Select "%windir%\System32\WindowsPowerShell\v1.0\powershell.exe" for your program name and set the full path to your powershell script as it's argument. Note that this will only run when a user logs in to the machine.

If instead you need this script to run without requiring a user to login, then goto Start -> Run -> Gpedit.msc. Under computer Configuration -> Windows Settings -> Scripts, double click on the "Start Up" icon to edit it's properties. Click on add and then the "Script Name" to be "%windir%\System32\WindowsPowerShell\v1.0\powershell.exe" and your script parameters to be the path to your powershell script. Hope this helps